This invention relates to a metallic urethane paint film having a high degree of geometric metamerism. More particularly, this invention relates to a high-solids paint composition containing isocyanate-terminated urethane resins and metal flake pigment, which resins are moisture curable to produce a decorative topcoat wherein the flakes are substantially uniformly oriented parallel to the topcoat surface to provide an aesthetically pleasing finish having a high degree of geometric metamerism.
Geometric metamerism is the ability of a paint film to exhibit subtle variations in color and appearance depending upon the directions from which it is illuminated and viewed. This effect is produced by metal flakes incorporated into the film at varying depths and lying substantially parallel to the film surface. Light rays traveling through the film are reflected by the flakes to the viewer. For a paint film also containing a color-producing pigment, the degree of interaction between that agent and a light ray--that is, the degree of color or hue--depends upon the distance traveled by the light ray through the film. Light rays reflected from flakes at varying film depths travel different distances through the film. Thus, the viewer perceives a combination of light rays having different degrees of color. More importantly, the distance traveled by light rays reflected off the flakes depends upon the angle of incidence and the angle of reflection. Light rays traveling normal to the flake surface travel a shorter distance than light rays traveling at acute angles. Thus, the "average" color is directly related to the illumination and viewing angles. The combination of these phenomena makes a film having geometric metamerism appear to be brighter and have a lighter hue when viewed normal to its surface than when viewed at an acute angle. That is, a viewer looking directly down upon the film surface observes a light, bright color. Flakes near the film surface reflect light that only slightly interacts with the color-producing pigment and so appear to sparkle. When the viewer shifts to look at the film at an acute angle to its surface, a darker, more intense color is perceived. Because of the greater degree of interaction, less sparkling is observed and the film appears generally deeper or thicker. At very shallow angles approaching the plane of the film surface, multiple reflections between metal flakes dramatically increase the light-pigment interaction and thereby makes the film appear extremely dark and deep.
Geometric metamerism, also referred to as goiniochromatism or simply "flop," makes an overall aesthetically pleasing impression upon the viewer. For this reason, it is highly preferred in decorative finishes and particularly in automotive topcoats. The subtle color variations helps to hide surface irregularities and highlight styling lines and contours. It is known that the degree of geometric metamerism is greatest when the metal flakes are oriented uniformly and substantially parallel to the film surface. More random metal flake orientation decreases the degree of geometric metamerism.
A typical prior art topcoat exhibiting a high degree of geometric metamerism is formed by the spray application of an aluminum flake-containing acrylic lacquer having a nonvolatiles content of between 15 and 35 weight percent. The viscosity of the low-solids lacquer is sufficiently low to permit the flakes to move and become approximately oriented as desired during film leveling immediately after application. The massive solvent evaporation during curing creates turbulence within the film and causes the film to shrink toward the substrate surface, thereby providing additional aligning forces to orient the flakes as desired. The result is a coating wherein the metal flakes are oriented substantially parallel to the film surface and thereby provide a high degree of geometric metamerism. Heretofore, the geometric metamerism obtained with low-solids lacquers has not been reproduced with paints having higher solids concentrations, such as 60 to 70%, because the higher viscosity and the lower solvent evaporation of such paints do not produuce the same uniformity of flake orientation.
It is also known that suitably formulated polyurethane resins provide a tough, yet flexible finish that has improved chip and impact resistance. Polyurethane paint compositions formulated with a high solids concentration are known to reduce solvent emissions during curing. While such properties recommend polyurethane paint binder resin for applications such as automotive topcoats, the use of such resins has heretofore been deterred because they did not display adequate resistance to degradation and fading when exposed to outdoor conditions and because they did not provide the good geometric metamerism obtained using low-solids lacquers. In my copending application, a polyurethane resin is disclosed that has an improved resistance to degradation and fading when exposed to moisture and sunlight.
Accordingly, it is an object of my invention to provide an improved polyurethane paint film that exhibits the high degree of geometric metamerism typically associated with metal flake-containing, acrylic lacquer finishes.
It is a further object of my invention to provide an improved paint composition containing an isocyanate-terminated, polyurethane prepolymer resin and aluminum flakes and having a high nonvolatiles concentration, said resin being moisture curable to produce a decorative coating wherein the metal flakes lie at varying depths and are oriented substantially parallel to the film surface to provide a high degree of geometric metamerism. The uniformity of flake orientation in the subject coating and, therefore, the degree of geometric metamerism are comparable to acrylic lacquer topcoats applied with significantly higher solvent concentrations.
It is a still further object of one aspect of my invention to provide a high-solids urethane paint composition that is moisture curable to produce a decorative topcoat having improved resistance to degradation and fading when exposed to outdoor conditions and also having a high degree of geometric metamerism. A urethane paint having these properties is particularly useful as an automotive topcoat.